Rotary drilling of oil wells and the like uses a drill bit, sometimes referred to as a rock bit, which is generally of the cone roller bearing type and may have tungsten carbide teeth or diamond bit inserts. The drill bits are generally made to a nominal diameter for cutting a hole. However, even this nominal diameter need not be precise and can have a tolerance depending on the type of drill bit of up to .+-.1/4 inch. The diameter of the drill bits are generally measured by conventional ring gauges. When drilling, wear takes place in the conical teeth of the drill bit regardless of the material. Thus when drilling a hole, the drill bit wears and thus the diameter of the drilled hole is reduced.
In the past it was found that in order to ensure a straight drill hole or to at least control the direction and angle of a drill hole, it was necessary to include a stabilizer. Stabilizers are positioned above the drill bit to ensure that the drill bit and stabilizer assembly drill a straight hole or else drill at an angle or curve that is determined by differences in diameter between the drill bit and the stabilizer. An example of straight hole drilling is shown in U.S. Pat. No. 4,874,045.
Stabilizers, whether a single stabilizer at the bottom of the stabilizer assembly or multiple stabilizers, generally all comprise spiral flutes and lands which are formed out of hardened carbon steel and are often integral with a bottom hole assembly in the form of a shaft with a tong arm neck above the lower stabilizer used to grip the assembly at the drill head for changing drill bits. It has been found that as that drill bit wears during drilling, the drilled hole reduces in diameter and at the same time the lands of the stabilizer which generally start off at being the nominal drill size being used, wear about the same amount otherwise they can become jammed in the hole. Thus, the drilled hole is smaller in diameter at the bottom than the nominal drill size or starting size.
When drilling deep holes it is often necessary to change drill bits. This is done by raising the drill string, gripping the bottom hole assembly around the tong arm neck and then removing the old drill bit and inserting a new drill bit. However, when the drill bit is lowered in the drill hole, as it gets closer to the bottom on the hole, it tends to jam as the hole is undersized. Thus, it has to be redrilled in order to increase hole size to the nominal size and this results in considerably more drilling time. Furthermore, at this stage, unless one has replaced the bottom hole assembly, the stabilizer is worn to a smaller diameter than the nominal drill size and this can result in not drilling a straight hole.
In most bottom hole assemblies, there is generally some distance between the drill bit and the stabilizer which can result in deviation from a straight drilled hole. If the stabilizer does not wear to the same extent as the drill bit then it acts to attempt to ream out the drill hole and this can cause the stabilizer to hang up or jam in an undersized hole.